1. Field of the Invention
The present invention relates to a shift-by-wire control system that performs range switching of an automatic transmission by using actuators, and more particularly to a system that performs adequately switching to a non-running range according to the vehicle state.
2. Description of the Related Art
In an automatic transmission such as a CVT or a planetary gear step AT provided at an automobile or the like, the drive, reverse, and neutral ranges are switched by controlling the oil pressure supplied to engagement elements such as a forward clutch and a reverse clutch.
Switching of such drive, reverse, and neutral ranges has conventionally been performed by a manual valve connected by a mechanical linkage to an operation lever operated by the driver.
A so-called shift-by-wire system, in which switching of running ranges is performed only by electric signals, without providing a mechanical linkage between the operation lever and the transmission, has recently been suggested.
As the conventional techniques relating to shift-by-wire systems of automatic transmission, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2008-128475 describes a range switching device in which spool valves are actuated by three respective solenoid valves to switch the oil pressure supplied to drive and reverse hydraulic servers.
Further, JP-A No. 2004-324849 describes a shift-by-wire system in which a park lock mechanism is automatically actuated when the ignition switch is OFF, wherein when the range of the automatic transmission is other than the P range in a state with the ignition switch being OFF, the range of the automatic transmission is set to the P range, regardless of the shift switch position, and when the ignition switch is switched from ON to OFF and the shift switch is in a range other than the P range, range switching of the automatic transmission is prohibited till the shift switch is in the P range.
Japanese Unexamined Utility Model Application Publication No. S64-17059 describes a shift-by-wire system in which a drive motor that switches gear ranges is controlled on the basis of signals from a gear indication switch, wherein a vehicle speed signal from a vehicle speed sensor and an ON/OFF signal of an ignition switch are inputted to a control circuit, and an automatic return to a parking range is performed when the vehicle is parked with the ignition switch being turned off.
From the standpoint of safety, it is undesirable that a vehicle in which shift-by-wire control is performed be driven when the driver has not buckled a seat belt or the driver's door is opened and the vehicle is not prepared for running.
Further, in the case where the voltage supplied to the system drops, a CPU or actuators can also malfunction and it is undesirable that the vehicle be driven is such a case.
When the automatic restart of the engine is prohibited for some reason after the engine has been automatically stopped in the idle stop control, usually even if the driver tries to start the engine manually, since the automatic transmission is in the running range, the inhibitor circuit is disconnected, the stator cannot be energized, and the engine cannot be restarted.
Further, even if the inhibitor circuit is set ON and the energizing of the stator motor is allowed as exception processing, it is possible that the start operation be performed in a running range.
The correct operation to be conducted in such a case involves performing an engine start operation after shifting temporarily to a no-running range, and then returning to the running range, but such an operation places a large load on the driver in emergency situations and the driver can fall into panic.